To effectively reduce a heavyweight-floor impact sound transmitted from an upper floor even when special specifications are not adopted in the case of the construction of a ceiling by lowering the rigidity of a ceiling board material.

Notches (2) (2)... for lowering the rigidity of the ceiling board material (1) are formed to at least one surface of a top face and an underside face. When a mass is reduced by the notches (2) (2)..., the reduction of the mass is made so as to hardly have an effect on the decrease of the floor impact sound by the lowering of the rigidity.